Ultrasound waves, currently used in medicine for prenatal scans and other diagnostic purposes, could one day be used as a noninvasive way to control brain activity. Over the past two years, scientists have begun experimenting with low-frequency, low-intensity ultrasound that can penetrate the skull and activate or silence brain cells. Researchers hope that the technology could provide an alternative to more-invasive techniques, such as deep-brain stimulation (DBS) and vagus nerve stimulation, which are used to treat a growing number of neurological disorders.
“Once people have found out what they can do with DBS and vagus nerve stimulation, we think we can unplug those devices and control activity from outside the body,” says William (Jamie) Tyler, a neuroscientist at Arizona State University, in Tempe. Tyler has started a company called SynSonix to commercialize the technology.
Devices designed to treat brain disorders have grown in popularity in recent years. DBS, which is used to treat Parkinson’s disease, dystonia, and obsessive-compulsive disorder, delivers an electrical jolt to the brain via an implanted electrode. Because of its invasive nature, however, DBS is only used for severe cases that are untreatable with medication. A less invasive technique is transcranial magnetic stimulation (TMS), in which an electric coil placed over the head generates a magnetic field that passes through the skull and excites neurons in the brain below. TMS is used to treat clinical depression, but it can only target the more superficial parts of the brain.
“With ultrasound, we have a much better spatial focus than [with] DBS,” says Tyler. “And unlike TMS, we can get anywhere in the brain.” Ultrasound–consisting of sound waves with a frequency above 20 kilohertz–has been used for decades in medicine to image muscle, organs, and fetuses. In the past five years, better tools for focusing ultrasound energy have enabled its use as an ablation tool: surgeons can now use high-intensity, high-frequency ultrasound (HIFU) to essentially burn away uterine fibroids. HIFU is also in clinical testing for treating brain tumors, breast tumors, and prostate cancer.
These same tools are now allowing scientists to apply ultrasound to control the brain, an idea that has actually been around for decades. Better ultrasound transducers, which generate the acoustic waves, enable more-precise focusing of ultrasound energy. And magnetic resonance imaging (MRI) used in conjunction with ultrasound allows surgeons to target specific areas of the body more precisely. “The ability to marry focused ultrasound with MR [magnetic resonance] guidance is exceedingly powerful,” says Neal Kassell, a neurosurgeon at the University of Virginia, in Charlottesville, and chairman of the Focused Ultrasound Surgery Foundation, a nonprofit based in Charlottesville that was founded to develop new applications for focused ultrasound.
One of the challenges in using ultrasound to target the brain is figuring out how to get the sound waves through the skull in a controlled manner. Typically, ultrasound operates in the megahertz to gigahertz range–frequencies that are fine for passing through soft tissue but would liquefy bone. (As bone absorbs the energy of the acoustic wave, it heats up.) Researchers at Brigham and Women’s Hospital, in Boston, have found that an ultrasound frequency of less than one megahertz can do the trick, but with a trade-off: the lower the frequency, the more difficult it is to focus the energy on a particular point in the brain.